The Iraqi government that the U.S. put into power during eight years of war lost the key city of Fallujah over the weekend. While you weren’t paying attention, al-Qaeda has returned to western Iraq with a vengeance, in the guise of the Islamic State of Iraq and the Levant.

Sunni insurgents seem largely in control of Iraq’s Anbar province, where an estimated 1,500 of the nearly 4,500 American troops killed in Iraq perished. Fallujah, the province’s second largest city, is the latest prize in the long-simmering war between the Shi‘ite and Sunni strains of Islam. The conflict has now come to a full boil, two years after the last U.S. troops, whose presence kept a lid on such internecine fighting, left Iraq.

Within hours of the city’s fall, Americans who fought or covered the pair of bloody 2004 campaigns to keep Fallujah out of Sunni militant hands expressed concern via the Internet over whether their fallen comrades had died in vain.

“Is this,” wondered Phillip Carter, an Army veteran of the Iraq War, who also served in a senior civilian role in the Obama Pentagon, “what it felt like for ’nam vets in ’75?”

“If you think Fallujah’s fall suddenly means your Iraq service was in vain, then you’ve been oblivious for 11 years,” added Brandon Friedman, who also served there as an Army infantry officer and wrote a book about the experience. “It was always pointless.”

“Sick about Fallujah,” tweeted James Garamone, a reporter for the Defense Department’s own press service. “I remember walking through the city when people started returning and believing that now they have a chance.”

Veteran Middle East observer Jeffrey Goldberg summed it up this way: “One war, from Beirut to Baghdad,” suggesting that the latest battle of Fallujah is simply the latest fight across a wide region of the globe between the Sunni and Shi‘ite sects.

Strangely, or perhaps not, the Pentagon was officially silent.

“This is a fight that belongs to the Iraqis,” Secretary of State John Kerry said on Sunday in Jerusalem. “That is exactly what the President and the world decided some time ago when we left Iraq. So we are not, obviously, contemplating returning. We’re not contemplating putting boots on the ground.” But he pledged unspecified U.S. help in returning stability to western Iraq.

Some Republicans were quick to pin at least some of the blame on the White House. “When President Obama withdrew all U.S. forces from Iraq in 2011, over the objections of our military leaders and commanders on the ground, many of us predicted that the vacuum would be filled by America’s enemies and would emerge as a threat to U.S. national-security interests,” GOP Senators John McCain of Arizona and Lindsey Graham of South Carolina said in a joint statement. “The thousands of brave Americans who fought, shed their blood, and lost their friends to bring peace to Fallujah and Iraq are now left to wonder whether these sacrifices were in vain.”

Outsiders, including Saddam Hussein, have always had trouble dealing with the restive city of 200,000, and foreign militaries have done no better. One reason: an errant British bomb during 1991’s Gulf War intended to take out a bridge over the Euphrates took out a market instead, killing about 100 civilians. Tensions spiked in March 2004 when Iraqi militants killed four U.S. contractors and hung their bodies from a bridge. That sparked two 2004 battles for Fallujah, during which about 100 U.S. troops were killed before the U.S. regained control.

The U.S. battles for Fallujah are seared into the memories of the troops who fought there because of their brutality, according to Marine Colonel John Ballard:

The fighting in Fallujah was up close, vicious, unpredictable and very manpower-intensive. Some buildings in the city were cleared multiple times. The tank was used often and with telling effect. Indirect fires from 155-mm artillery positioned less than 5 km away in Camp Fallujah were used on a daily basis before, during and after the heaviest period of fighting. Frequently, small-unit leaders would push ‘stacks’ of Marines or soldiers into buildings while employing laser-guided bombs, artillery and tank main-gun rounds on adjoining structures. The combat bulldozer was used by combat engineers and Seabees on several occasions to push the walls of buildings in on stubborn defenders. Insurgents used armor-piercing bullets and even sewed grenades inside their clothing to kill and maim U.S. troops at any opportunity.

The Iraqi government pledged on Sunday to retake the city in the coming days in a counteroffensive to be led by local tribes instead of the Iraqi military. That’s because Sunni lawmakers see the army — trained at a cost of $25 billion by U.S. taxpayers — as Prime Minister Nouri al-Maliki’s cudgel to eliminate Sunni rivals and consolidate power.

Would it have turned out differently if U.S. forces were still in Iraq (the last GIs left on Dec. 18, 2011, after Iraq refused to grant legal immunity to any remaining U.S. troops)? Perhaps, at greater loss in U.S. blood and treasure. But Obama decided that, after more than eight years of war there, the U.S. public was ready to toss in the towel. Granted, he could have twisted additional Iraqi arms to try to get such legal protections, but the voters who had put him into office three years earlier weren’t interested.

Todd Bowers was a Marine who served in Fallujah during some of the toughest fighting (that’s his video from the city below). Who lost Fallujah? “We all did,” he said on Sunday. “I guess we all just decided it was easier to forget Fallujah and get on with life. Vets, politicians and the general public.”

As the third battle for Fallujah looms, a passage from a 2011 Pentagon probe into the U.S. effort to build a new sewer system for the city could stand, in miniature, as the key lesson of the entire war:

“Little planning went into the project, and there was minimal understanding of site conditions, no skilled workforce available, and no clear idea about how much the new system would cost,” the investigation concluded. “So many adverse conditions faced this project from the outset; thus, it is hard to understand why it was initiated and continued.”